Field of the Invention
A method of operating an inductive power transfer system and an inductive power transfer system
Description of Related Art
The invention relates to a method of operating an inductive power transfer system for transferring power to a vehicle and such an inductive power transfer system.
Vehicles, in particular electric vehicles, more particular track-bound vehicles, and/or a road automobile, can be operated by electric energy which is transferred by means of an inductive power transfer. Such a vehicle may comprise a circuit arrangement, which can be a traction system or a part of a traction system of the vehicle, comprising a receiving device adapted to receive an alternating electromagnetic field and to produce an alternating electric current by electromagnetic induction. Furthermore, such a vehicle can comprise a rectifier adapted to convert an alternating current (AC) to a direct current (DC). The DC can be used to charge a traction battery or to operate an electric machine. In the latter case, the DC can be converted into an AC by means of an inverter.
The inductive power transfer is performed using two sets of e.g. three-phase windings. A first set is installed on the ground (primary winding structure) and can be fed by a wayside power converter (WPC). The second set of windings (secondary winding structure) is installed on the vehicle. For example, the second set of windings can be attached underneath the vehicle, in the case of trams under some of its wagons. For an automobile, it can be attached to the vehicle chassis. The second set of windings or, generally, the secondary side is often referred to as pick-up-arrangement or receiving device. The first set of windings and the second set of windings form a high frequency transformer to transfer electric energy to the vehicle. This can be done in a static state (when there is no movement of the vehicle) and in a dynamic state (when the vehicle moves).
Known are inductive power transfer systems which comprise a movable primary element. U.S. Pat. No. 5,654,621 A discloses an inductive transmitter having a primary element and a secondary element which is attached to the vehicle, wherein the primary element is power driven to move in all three spatial coordinates with a predetermined spatial area.
DE 102010042395 A1 discloses a system for inductive charging of a battery of a vehicle, wherein a primary coil is automatically placeable.
DE 102007033654 A1 discloses a base unit with a driving means to reduce a distance between a primary conductor and a secondary coil.
US 2010/0235006 A1 discloses a movable automated charging apparatus comprising a base, a scissor lift, a pedestal, a joint and a charger. The charger is configured to mate with a vehicle receptacle physically or via proximity.
GB 1403547.1 (not yet published) discloses an inductive power transfer pad comprising a stationary part and a movable part, wherein the movable part comprises a primary winding structure, wherein the movable part is movable between a retracted state and an extended state.
PCT/EP2013/067414 discloses an inductive pick-up arrangement to be mounted on an electric vehicle which shall be operated with electric energy produced by the arrangement by magnetic induction, wherein:                the arrangement comprises a pick-up portion comprising at least one electric inductance for receiving a magnetic field and for producing the electric energy,        the arrangement comprises a mounting portion to be mounted on the vehicle,        the arrangement comprises one actuator or a set of at least two actuators for actuating movement of the pick-up portion relative to the mounting portion,        the mounting portion and the pick-up portion are moveably connected to each other by at least one connecting portion,        the actuator or the set of actuators can be actuated such that the pick-up portion is moved in a vertical direction,        the actuator or the set of actuators can be actuated such that the pick-up portion is additionally or alternatively moved in a lateral direction.        
Further disclosed is that a movement of the pick-up portion is controlled in dependence of an output voltage of an electric inductance mounted on the pick-up portion.
FIG. 1 shows a first layout of an inductive power transfer system 1 according to the state of the art. A wayside WS and a vehicle side VS are indicated. A direct current (DC) source provides a DC output voltage, typically in the range of 500 V to 900 V. The DC voltage source provides the output voltage to an input voltage generating means 3 which can generate an output voltage higher than its input voltage. The output voltage of the input voltage generating means 3 is provided to a wayside power converter (WPC) 4 which comprises an inverter 5 and an output filter 6.
The input voltage generating means 3 is shown in FIG. 2. It comprises a step-up converter 7, an intermediate circuit capacitor 8 and a step-down converter 9 connected in series. The step-up converter 7 generates a voltage which is higher than its input voltage, e.g. the output voltage of the DC voltage source 2 (see FIG. 1). The step-down converter 9 generates a variable DC output voltage which, in turn, can provide the input voltage of the WPC 4. Thus, the input voltage generating means 3 is provided by a two-stage voltage converter.
Returning to FIG. 1, the WPC 4 generates an input voltage for a high frequency transformer 10 which comprises the primary winding structure and the secondary winding structure (not shown). The secondary winding structure of the high frequency transformer 10 provides an alternating current (AC) output voltage which is rectified by a rectifier 11, wherein a rectified output voltage of the rectifier 11 is provided to an energy storage element 12, e.g. a battery or an accumulator, or to a network of the vehicle, in particular a traction network, for example to a DC voltage link of the network.
FIG. 3 shows a layout of another inductive power transfer system 1 according to the state of the art. The inductive power transfer system 1 comprises an AC voltage source 13 which provides an AC voltage to an input voltage generating means 3 (see FIG. 2). An output voltage of said input voltage generating means 3 provides the input voltage for a WPC 4. The remaining elements, namely the high frequency transformer 10, the rectifier 11 and the energy storage element 12 are connected to the WPC 4 as explained with reference to FIG. 1.
In the layout according to FIG. 3, the input voltage generating means 3 rectifies the AC input voltage, wherein step-up converter 7 generates an output voltage and (in contrast to the layout shown in FIG. 1) provides a correction of the power factor.
The adjustment range of the output voltage of the step-down converter 9 of the input voltage generating means 3 has to cover the complete charging voltage range of the energy storage element in order to charge energy storage element 12 from a minimal voltage Umin to a maximal voltage Umax or the complete operating voltage range of the aforementioned network of the vehicle. This, however, is only providable by additionally using the step-up converter 7 in the aforementioned two-stage layout since the step-up converter 7 allows varying an input voltage of the step-down converter 9.
Further, an input current of the energy storage element 12 can be limited, e.g. depending on a state of charge of said energy storage element 12. The step-down converter 9 controls its output voltage depending on an actual input current of the energy storage element 12 such that the maximal input current is not exceeded.
The two-step layout of the input voltage generating means 3 shown in FIG. 2 and used in the layouts shown in FIG. 1 and FIG. 3 has the disadvantage that each step, namely the step-up conversion by the step-up converter 7 and the step-down conversion by the step-down converter 9 generate power losses. Such losses increase in the case that voltage levels of the DC input voltage of the step-up converter 7 and the DC output voltage of the step-up converter 7 are different.